Naturally occurring bromine molecules, br2 have masses of 158, 160, and 162. they occur in the relative abundances 25.69%, 49.99%, and 24.31% respectively. what is the average atomic mass of bromine atoms? what is the relative abundance of 79br and 81br isotopes?

The average atomic mass of an element can be determined by multiplying the individual masses of the isotopes with their respective relative abundances, and adding them. 

Average atomic mass of Br = 158 amu(0.2569) + 160 amu(0.4999) + 162 amu(0.2431)
Average atomic mass = 159.96 amu

As described in the problem, the relative abundance for Br-79 is 25.69%. This is because 2 atoms of Br is equal to 79*2 = 158 amu. Similarly, the relative abundance of Br-81 is 81*2 = 162, which is 24.31%.

D. Non-metals tend to have high ionization energy and high electronegativity.​

Explanation:

non-metals have to gain electrons, so they have more valence electrons than metals. Therefore, it requires more energy to remove them. ... When a metal loses an electron, a nonmetal gains an electron. So, non-metals have high ionization energy.

Nonmetals have more valence electrons and increase their stability by gaining electrons to become anions. The electronegativities of nonmetals are generally high.